1. Field of the Invention
This invention relates to a power multiplication circuit using a Hall element, and more particularly to a power multiplication circuit used in, such as a power meter or a watt hour meter, wherein the offset compensation means of the Hall element is improved.
2. Detailed Description of the Related Art
In general, power multiplication circuits of this type are widely used as power multiplication circuits of power meters and/or watt hour meters on account of their uncomplicated construction and small size. Such a conventional power multiplication circuit will now be described with reference to FIG. 4. The power source voltage of a system under measurement is input at input terminals P1, P2, passes through a voltage conversion circuit 1 comprising a voltage divider circuit with resistors RA and RB, and is input to a voltage-current conversion circuit 2. Voltage-current conversion circuit 2 outputs a current proportional to the input voltage to control current terminal T1 of Hall element 3. Meanwhile, the current from the system under measurement is input to terminals 1S, 1L of current coil 105 would on magnetic material core 104 shown in FIG. 5, so that a magnetic field proportional to the input current is generated in gap 106. As a result, a Hall electromotive force Ey given by expression (1) is generated at voltage output terminals T3, T4 of Hall element 3, which is positioned so as to be at right angles to the magnetic field of gap 106 and to the direction of flow of the control current of Hall element 3 which is from terminal T1 to terminal T2. EQU Ey=Rh.multidot.Bz.multidot.Jx (1)
where Rh is the Hall coefficient, Bz is the magnetic field intensity, and Jx is the current density. In FIG. 4, the direction of the magnetic field is shown as an arrow x. Variable resistor VR1 is connected between voltage output terminals T3, T4 and serves to compensate for the offset voltage generated by the asymmetry of the characteristic of Hall element 3. Its moving contact is connected to ground together with control current terminal T2. Output terminals OUT1 and OUT2 of the power multiplication circuit are connected to voltage output terminals T3 and T4, respectively.
Next, the mechanism of offset voltage generation will be described with reference to the internal equivalent circuit of Hall element 3 shown in FIG. 6. The equivalent circuit is expressed by a resistance bridge circuit, as shown. The output voltage is given by expression (2), if the respective voltages at the nodes are taken as E1, E2, E3, and E4 when the voltage E0 is applied to terminal T1 and the bridge resistances are respectively R1, R2, R3 and R4. ##EQU1##
If the magnetic field is 0, EQU R1/R2=R3/R4 (3)
So, E3-E4=0.
Though, in the above expressions (2) and (3), the voltage E0 and resistances R5, T6, R7 and R8 are not expressed positively, they are included in the voltages E1, E2, E3 and E4. And when Hall element 3 is connected to a following circuit having a very high input impedance, the output voltage between terminals T3 and T4 of Hall element is nearly equal to the voltage (E3-E4) expressed in the expression (2).
However, if for example resistance R1 fluctuates in the direction such that it is decreased by an amount r1, ##EQU2##
So, an offset voltage is generated between terminals T3 and T4 by the residual voltage even if the magnetic field is 0.
Next, the effect of the offset voltage when a magnetic field is present will be described. We assume that a magnetic field as shown in the Figure is applied to Hall element 3 of FIG. 6, and that the resistances of resistors R1, R4 decrease while the resistances of resistors R2, R3 increase. The result of expressing graphically the potential difference between output voltage terminals T3, T4 is shown in FIG. 7 when a half-wave AC current flows between terminals T1, T2 for the four combinations of "positive"/"negative" direction of the magnetic field and "0 degrees/"180 degrees" of the phase of the control current. The upper row of diagrams in FIG. 7 indicates the waveform of the magnetic field intensity. The middle row indicates the waveform of the power source voltage of the system under measurement. The lower row indicates the waveform of the output voltage which is shown by broken lines in the case where there is no offset. If now we assume that the offset of resistor R1 is generated in the direction of decreasing resistance, the output voltage is decreased or increased by the amount of the offset voltage, as shown by the continuous line in the bottom row of FIG. 7. Accordingly, the output voltage is adjusted to zero in the condition where magnetic field is not applied by providing a variable resistor VR1 as shown in FIG. 4 to cancel the deviation of the output voltage produced by this offset voltage.
In the conventional power multiplication circuit described above, adjustment of the variable resistance in order to compensate the offset for fluctuations in the offset resistance produced by temperature change or occurring over a period of years was indispensable. However, for watt hour meters used by ordinary users this type of continuous adjustment operation is impracticable and occurrence of errors was therefore inevitable. Due to the need for a component having a mechanical contact part such as a variable resistor, there was also a reliability problem, and the power multiplication circuit was difficult to implement as an LSI.